The principal goal of the studies proposed in this application is to elevuate change during aging in a specific set of neuronal cytoskeletal elements, the microtubules and tubulin of the axon, which are important in the generation and maintenance of neuronal morphology. The ability of the neuron to produce cellular processes many times larger than the neuronal cell body and then to maitain the fundamental forms of these processes for the life of the organism is essential for the function of the nervous system. Changes inthe properties of cytoskeletal elements that form the basis of neuronal morphology are likely to be reflected in the morphology. The use of axonal transport processes as a paradigm to specifically label axonal tubulin led to my discovery that axonal tubulin has properties that are significantly different from the tubulin obtained in cold extracts of whole brain. The bulk of the axonal tubulin is cold-stable and biochem-distinct from cold-extractable tubulin of whole brain. Age-related changes in the morphology, organization, and plasticity of neurons have been noted and all of these parameters are considered to be affected by tubulin and microtubules. My discovery that axonal tubulin is largely cold-stable raised the question: Do changes during aging in the properties of axonal tubulin underly age-related changes in the properties of neuronal morphology and plaxticity? In an effort to answer this question, the following plan of research is proposed: 1) Purification procedures and an assay for quantitative analysis of axonal tubulin is to be devised using tubulin labelled by axonal transport. The specificity of axonal transport for labelling axonal tubulin facilitates analysis. 2) The biochemical basis of cold-stability in axonal tubulin will be evaluated. This should provide insight into the molecular mechanisms of modulating the properties of cytoskeletons. 3) The amount and the properties of axonal tubulin will be determined in rats aged 3-26 months. Changes in the fraction of axonal tubulin that is cold-stable can be correlated with age-related changes in rate of axonal transport and the capacity of neurons to regenerate axons. These studies should provide a better understanding of the molecular mechanisms by which aging changes in the properties of the nervous system are mediated and of the roles that tubulin plays in the structure and functions of the axons.